The disclosure relates generally to fuel cells, and more particularly to the management of liquid for a fuel cell power plant. More particularly still, the disclosure relates to liquid level detection and control for a fuel cell stack assembly for a fuel cell power plant.
Fuel cell power plants convert chemical energy into usable electrical power. Fuel cell power plants typically comprise multiple fuel cells arranged to form a fuel cell stack assembly (CSA), including internal ports or external manifolds that connect coolant fluid and reactant gas flow passages or channels. An individual fuel cell in a CSA typically includes an electrolyte, such as a proton exchange membrane (PEM), interposed between an anode catalyst layer (anode) and a cathode catalyst layer (cathode) to form a membrane electrode assembly (MEA). Directly on either side of the MEA are porous gas diffusion layers (GDL) followed by reactant flow field plates that can be of gas permeable porous construction or can be solid with defined channels therein. These plates supply a reactant fuel (e.g. hydrogen) to the anode, and a reactant oxidant (e.g. air or oxygen) to the cathode. Protons formed at the anode are selectively transferred via the membrane (PEM) to the cathode. The electrons formed at the anode serve to produce an external electrical current, and are recombined with protons at the cathode, resulting in the further production of water and thermal energy.
Fuel cell power plants may comprise subsystems for dealing with the management of water and the thermal energy produced. The electrochemical reaction in a fuel cell is more efficient at certain operating temperatures, and overheating can cause drying out of the PEM, which not only hinders or prevents the electrochemical reaction from occurring but also can lead to physical degradation of the membrane itself. Conversely, excessive moisture in the CSA can also lead to performance degradation when product water formed at the cathode, for example, accumulates and blocks reactants from reaching the PEM surface, thus inhibiting the electrochemical reaction.
In order to address the problems of excessive heat, drying, and moisture associated with fuel cells, various systems have been developed for carefully managing the fluid balance in the CSA such that it stays sufficiently cooled and hydrated for maximum stack performance. Regardless of the system used, the coolant fluid, typically a liquid, must be uniformly distributed throughout the CSA via a fluid flow path in order to prevent the formation of thermal gradients and/or to properly humidify the reactants. Consequently, various techniques have been employed in the art to verify whether a proper liquid balance is present in the CSA, including monitoring coolant flow and overall fluid levels. Since the fluid flows and levels of concern herein typically involve a liquid, the term “liquid” will be generally be used hereinafter with respect to that medium of concern, though it will be understood that fluids in gas phase are also present and flow in the CSA.
As an example, in systems where liquid collects in a reservoir, overall liquid presence in the CSA can be measured as a function of a height of a column of liquid in the reservoir by using a float type sensor. However, such sensors are comprised of mechanical parts that are subject to breakage over time, and are further prone to giving false readings under frozen conditions.
Other systems detect the presence of liquid by using a conductivity sensor in contact with the liquid. These systems have the limitation or disadvantage of requiring the generation of a primary signal from either a battery or wire tap off of the external circuit, and thus increase the complexity and/or weight of the system and/or decrease the amount of power available to the primary load.